refactor: replace border-box with box-border and clean up unused scss

Remove the unused box-sizing.scss stylesheet and its import from the main SCSS index file.
Update all Vue component template class references from the custom border-box class to the
native box-border utility class to eliminate redundant custom CSS and align with the project's
utility class conventions.

src/components/TopNavBar/index.vue

@@ -4,7 +4,7 @@
     <view :style="{ height: statusBarHeight + 'px' }" v-if="!hideStatusBar"></view>
 
     <!-- 导航栏内容 -->
-    <view class="flex flex-items-center flex-justify-between border-box pl-[8px] pr-[8px]" :style="navContentStyle">
+    <view class="flex flex-items-center flex-justify-between box-border pl-[8px] pr-[8px]" :style="navContentStyle">
       <!-- 左侧返回按钮 -->
       <view class="nav-bar-left flex flex-items-center flex-justify-center" v-if="showBack" @click="handleBack">
         <uni-icons type="left" size="20" :color="backIconColor" />